In recent years, the so-called function separated type electrophotographic photoreceptor is developed and subjected to practical use from the standpoint of sensitivity and stability, in which a charge generating layer and a charge transporting layer are separated from each other. The electrophotographic photoreceptor having such a constitution contains two layers, i.e., a layer having a charge generating substance bound with an appropriate resin as a binder and a layer having a charge transporting material dispersed or dissolved in a binder resin. The layer containing the charge transporting material contains, in many cases, a hole transporting material, and a thermoplastic resin, such as a polycarbonate resin, a polyester resin, an acrylic resin and a polystyrene resin, and a thermosetting resin, such as a polyurethane resin and an epoxy resin, are investigated as a binder therefor. In this case, the surface of the charge transporting layer should be charged negatively by corona discharge or roller discharge, and there is a problem in that the photoreceptor characteristics are deteriorated due to various factors, in which deterioration of the resin caused by ozone formed thereon, wear-out and deterioration in sensitivity and charging ability caused by an electric shock due to discharge carried out on the surface of the photoreceptor, and mechanical breakage caused by friction on development with a toner, transfer to paper and cleaning.
Various studies have been conducted to solve the problem. For example, the following methods are proposed, i.e., a method described in JP-A-1-161279, in which an abrasion device of an electrophotographic photoreceptor is provided on an ordinary photoreceptor and used at an abrasion amount of from 1 to 1.5 μm per 10,000 sheets of duplication, whereby contamination substances on the surface are removed; a method described in JP-A-6-75384, in which a photoreceptor is used at an ozone concentration around the photoreceptor of from 5 to 50 ppm and an abrasion amount of 300 Å per 1,000 revolutions; and a method described in JP-A-7-311470, in which the pressing force of a cleaning blade onto the photoreceptor is specified to a particular value, which is used at an abrasion amount on the cleaning process of from 0.05 to 1.0 μm per 1,000 cleaning cycles, and a releasing agent having a number average domain diameter of from 0.1 to 1.1 μm is added to a toner. However, these methods can be applied to a non-contact charging process, such as corotron and scorotron, but in a contact charging process represented by the roller charging, it causes a problem in that the charging stress is large to increase the wear rate, which substantially cannot be controlled, so as to shorten the service life of the photoreceptor, and thus an outermost layer having higher strength becomes necessary.
In order to improve the strength of the outermost layer, it has been studied to provide various overcoat layers. In particular, it has been tried that a polysiloxane resin is used as a copolymer component or is mixed with other resins, described in JP-A-61-238062, in which a thermosetting resin containing a polysiloxane resin is used in a charge transporting layer, in JP-A-62-108260, in which a protective layer containing a polysiloxane resin is provided, in JP-A-4-346356, in which silica gel, a urethane resin and a fluorine resin are dispersed in a thermosetting polysiloxane resin, which is used as a protective layer, and in JP-A-4-273252, in which a resin obtained by dispersing a thermosetting polysiloxane resin in a thermoplastic resin is used as a protective layer or a binder resin for a charge transporting material, whereby improvements in performance, service life and cleaning property of the photoreceptor are studied by utilizing the characteristics of polysiloxane.
However, because polysiloxane is extremely poor in compatibility with an organic compound although it has characteristics, such as transparency, dielectric breakdown resistance and photostability, as well as a low surface tension, which are not found in other resins, it is not used solely as a resin for constituting a charge transporting material but is used for modifying a resin for constituting a charge transporting material by copolymerization or blending. In order to use the polysiloxane resin solely as a binder for constituting the charge transporting layer, it is necessary to find out a charge transporting substance that can be dissolved in the polysiloxane resin.
Therefore, the following studies have been made, i.e., a resin obtained by directly bonding a charge transporting agent having an unsaturated bond to polysiloxane, such as poly(hydrogenmethylsiloxane), by hydrosililation is used as a protective layer or a binder resin for a charge transporting material (described in JP-A-8-319353), an inorganic thin film formed by plasma CVD is used as a protective layer (described in JP-A-7-333881), a thin film formed by a sol-gel process is used as a protective layer (described in Proceedings of IS&T's Eleventh International Congress on Advances in Non-Impact Printing Technologies, pp. 57 to 59), and an organic silicon modified hole transporting compound formed by directly introducing a silicon group having a hydrolytic group to a charge transporting agent is used in an electrophotographic photoreceptor (described in JP-A-9-190004) Among these, those described in Proceedings of IS&T's Eleventh International Congress on Advances in Non-Impact Printing Technologies, pp. 57 to 59, Japanese Patent No. 2,575,536 and JP-A-9-190004 receive attention because siloxane forms a three-dimensional network to form a firm film, and thus the mechanical strength is greatly improved.
A photoreceptor having such a layer having a high strength as, for example, an overcoat layer is liable to form image defects due to surface contamination, since the wear amount thereof is generally small. Therefore, the following methods have been proposed, i.e., a protective layer of a high strength is provided on an organic photoreceptor, which is used at a wear amount of from 7 to 200 Å (from 0.0007 to 0.02 μm) per 10,000 sheets of duplication as described in JP-A-1-133086, and it is used with wearing in an amount of from 0.001 to 0.010 μm per 1,000 revolutions as described in JP-A-7-225541. Because a hard layer having a charge transporting material introduced into a three-dimensional network of siloxane by a chemical bond becomes a semiconductor, it is advantageous that horizontal diffusion of charge can be prevented with respect to an insulating layer, an ion electroconductive layer or a surface protective layer formed with a resistance controlling layer having electroconductive powder dispersed in a dielectric polymer, so as to form a stable image.
However, in the case where a surface layer containing a three-dimensional network of siloxane having a charge transporting material contained therein is employed, because siloxane contains a large amount of unreacted hydroxyl groups as the nature thereof, a product of corona discharge and moisture are liable to be adsorbed on the surface thereof. Furthermore, the mechanical strength thereof is larger than that of the ordinary charge transporting layer by 10 to 20 times, and thus the adsorbed substances are liable to remain for a long period of time.
Therefore, it has a problem in that a reaction between the charge transporting material and the adsorbed substances is liable to occur, and the reacted and deteriorated charge transporting material remains on the surface layer to frequently cause image defects. Thus, development of an apparatus for forming an image that solves the problem to realize a long service life is demanded.